Electrolytic polymerization of aromatic compounds



United States Patent 3,437,570 ELECTROLYTIC POLYMERIZATION 0F AROMATICCOMPOUNDS Norvell E. Wisdom, Jr., Elizabeth, N.J., assignor to EssoResearch and Engineering Company, a corporation of Delaware No Drawing.Filed Dec. 11, 1967, Ser. No. 689,350 Int. Cl. B01k 3/00 US. Cl. 204-598 Claims ABSTRACT OF THE DISCLOSURE Polymeric aromatic compounds areprepared by electrolyzing a liquid solution of a C to C aromaticcompound, such as biphenyl, and a ternary complex having the formulaRzHXz2AlX where R is a C to C aromatic compound at least as basic as thearomatic in solution and X is chlorine or bromine in which the.potential of the working anode (vs. the saturated Cu:Cu Cl is 1.15 forthe first two to ten minutes and then is 1.05 to 1.10 and in which theAlCl is preferably contained in a porous wall container.

Cross reference This case is related to Ser. No. 549,482, filed May 12,1966 by David G. Walker and Norvell E. Wisdom, Jr.

Background This invention relates to a process of electrolyticallypolymerizing and oligomerizing aromatic compounds and more particularlyrelates to methods for improving current efficiency in such a process.

In the application Ser. No. 549,482, supra, there is described andclaimed a process for the electrolytic polymerization of C to C aromaticcompounds, such as benzene, toluene, xylene, biphenyl, mesitylene, andthe like in which the electrolyte is a ternary complex having theformula RzHXz2AlX where R is a C to C aromatic compound at least asbasic as the aromatic compound being polymerized and X is chlorine orbromine.

The efficiency of the current for this synthesis has been found to varyfrom 10 to 100% depending upon the conditions (calculated on the basisof a requirement of two electrons for each original aromatic moleculeincorporated into the polymer in the center of a chain and one electronfor each such molecule on the end of a chain. It has also been foundthat the se variations are due at least in part to a complex variationof the potential of the anode at which synthesis occurs i.e. the workingelectrode, with current density and time. However, no polymer can beformed at an anode whose potential is kept too low throughout the courseof an experiment.

Summary In accordance with this invention it has been found that acurrent efficiency of 90 to 100% in the electrolytic polymerization ofaromatic compounds using R:H:2AlX as the electrolyte may be reproduciblyachieved by making the potential of the working anode 1.15 volts (vs.the saturated Cu:Cu Cl electrode.) for two to ten minutes at thebeginning of the electrolysis, then reducing the electrode potential to1.05 to 1.10 volts (vs. the saturated Cu:Cu Cl electrode) for theremainder of the electrolysis. The potential variation required can beadvantageously and automatically accomplished by controlling the currentdensity of the working electrode. The saturation of the electrolyte,specifically the anolyte, is maintained by pro viding a porous walledreservoir within the anode for holding solid aluminum chloride.

Preferred embodiments Thus, in accordance with the invention, describedin Ser. No. 549,482, high yields of polymeric aromatic hydrocarbons areprepared by electrolyzing a solution comprising a C to C aromaticcompound and a ternary complex having the formula R:HX:2AlX wherein R isa C to C aromatic compound at least as basic as the aromatic to bepolymerized, and X is selected from the group consisting of chlorine andbromine. The overall electrolytic reaction may be illustrated, withrespect to the preparation p-sexiphenyl, by the following expression:

3C H (biphenyl) C H (p-sexiphenyl) +2H (1) with p-sexiphenyl beingproduced at the anode and hydrogen gas at the cathode.

The ternary complex used in this invention has many interestingproperties, among which two are particularly important to this process:(1) the ability to exist as an ionic phase according to the followingequation:

which imparts a relatively high degree of electrical conductivity to theternary complex, thereby allowing the ternary complex to function as theelectrolyte in the electrolysis reaction; (2) the. ability to dissolvesubstantial amounts of aromatic compounds, over and above the amountrequired to form the ternary complex. The dissolved aromatic compoundsmay be referred to as excess aromatics and the solution produced therebywill be referred to as the complex phase.

The polymers which may be formed by utilizing the process of thisinvention are quite varied and dependent upon the material used as theexcess aromatic in solution. Generally, the excess aromatic may be a Cto C aromatic compound. Preferably, the aromatic may be selected fromthe group consisting of benzene, biphenyl, naphthalene, alkylsubstituted benzenes, naphthalenes, naphthalenes, and biphenyls, andhalo derivatives thereof,

the hydrocarbon compounds being preferred. While alkyl (llH 0 H I C H; CH;

from mesitylene, and various dimers, trimers, and higher oligomers fromferrocene o-xylene m-xylene p-xylene, 1,2, 4-trimethyl benzene,l,2,4,5-tetramethyl benzene, chlorobenzene and the like. The polymersproduced herein are normally characterized by the loss of hydrogen atthe coupling site.

The ternary complex which functions as the electrolyte in this processis represented by the formula:

wherein R is a C to C aromatic compound at least as basic, andpreferably more basic than the aromatic to be polymerized. Preferredaromatic compounds are selected from the group consisting of benzene,biphenyl, naphthalene, alkyl benzenes, naphthalenes, and biphenyls, andhalo derivatives thereof, preferably a hydrocarbon, more preferably C toC alkyl benzenes, and still more preferably C to C alkyl benzenes; and Xis selected from the group consisting of chlorine and bromine. Thebasicity of a compound, as used herein, designates the tendency of thatcompound to accept a proton, i.e. the greater the basicity, the greaterthe tendency to accept a proton. Illustrative of the aromatichydrocarbons which may be used as R in the ternary complex and listed inthe order of increasing basicity are: benzene, biphenyl, toluene,xylene, pseudocumene, hemimellitene, durene, mesitylene, prehnitene,isodurene, pentamethylbenzene, hexamethylbenzene. Other compounds whichalso may be used are: isopropyl benzene, l,3,S-dimethylethylbenzene, theethyl toluenes, methylnaphthalene, dimethylnaphthalene, ethylbenzene. Areview of the relative basicities of methylbenzenes and the method usedfor determining basicity is presented in Ehrenson, J. Am. Chem. Soc. 84,2681- 2687 (1962). For example, when biphenyl is the excess hydrocarbon,R may be biphenyl but preferably is more basic, e.g. toluene. However,more highly substituted alkyl benzenes, i.e. the C to C alkyl benzenes,are normally preferred in p-sexiphenyl production.

The ternary complex may be prepared in substantially pure form by mixinga suitable aromatic compound, as described above, with a stoichiometricexcess of HCl or HBr and an aluminum halide, i.e. AlCl AlBr at atemperature between 50 C. and +30 C. A preferred method for preparingthe ternary complex consists of mixing the aromatic compound with ananhydrous aluminum halide powder at room temperature. The mixture isstirred and anhydrous HCl or HBr is allowed to bubble through themixture. It is necessary to provide a stoichiometric excess of both thehydrogen halide and aluminum halide to insure that all of the aromaticcompound will be reacted. (The use of less than a stoichiometric amountof aluminum halide will tend to the formation of monomer complexes,wherein the aromatic2hydrogen halidezaluminum halide ternary compoundwill form in the mole ratio of 1:1:1. Impure compounds with an HXzAlXratio of less than 1:2 may also form, but are not desirable in theprocess of this invention. The monomer complexes are not applicable tothe process of this invention, unless restricted to the cathode chamberonly of a cell with a porous diffusion barrier between anode andcathode. Electrolysis of an aromatic saturated monomer complex yieldshydrogen evolution at the cathode and chlorine evolution at the anodealong with the formation of chlorinated products at the anode.)Substantially pure ternary complexes prepared by either of the foregoingprocedures will be saturated with respect to hydrogen halide andaluminum halide; however, the presence of these compounds at saturationwill be small and will not be detrimental to the process of thisinvention.

The complex phase may be readily prepared by mixing excess aromatic withthe ternary complex. Since the ternary complex is capable of dissolvingexcess aromatics, the complex phase will comprise a solution of excessaromatic and ternary complex. Normally, the ternary complex is capableof dissolving about five to seven moles of excess aromatic beforesaturation, depending upon the excess aromatic employed. However, in thecase of hiphenyls or naphthalenes, the ternary complex will dissolveonly about three moles of excess aromatic. In ordinary circumstances thecomplex phase should comprise at least 0.5 mole, and preferably 1.0 moleof excess aromatic per mole of ternary complex. Particularly preferred,however, is a ternary complex saturated with excess aromatic. Additionof excess aromatic above that required to form a saturated complex phasewill not be deleterious, but will not enter into the electrolysisreaction since a separate non-conductive phase containing the excessaromatic will form. As noted, the ternary complex is capable ofdissolving excess aromatic hydrocarbons. Therefore, the preparation ofthe complex should be carried out to keep amount of excess R, i.e. overand above that required to form the ternary complex, to a minimum,thereby avoiding undesirable side reactions during electrolysis.

The electrolysis may be carried out in any suitable type of cell, eitherwith or without a diffusion hindering membrane. The aromatic polymerwill for-m at the anode, or in the anode compartment when a membrane isemployed. The anode is preferably selected from the platinum groupmetals, i.e. platinum, palladium, rhenium, ruthenium, osmium, iridium,or tantalum. Platinum, however, is particularly preferred. The cathodemay be of any convenient material, e.g. aluminum, carbon; however, theplatinum group metals are also preferred for the cathode. It is alsopossible, and in some instances economically desirable, to utilize basemetals as the electrodes. When using base metals, they are preferablyplated with one of the platinum group metals.

Diffusion hindering membranes may be utilized, if desirable. In general,such membranes may be of any material that is chemically inert to theternary complex and will form a diffusion barrier while allowing iontransfer. Examples of such membranes are numerous, among which are:fritted glass, sintered glass, asbestos, porous ceramics, e.g. Alundum,zirconia, porous plastics, e.g. cellophane, paper products, e.g.parchment, perforated metals, and the like. When a diffusion hinderingmembrane is used, the ternary complex can be used in both compartmentsto function as the electrolyte. However, if desirable, the ternarycomplex need only be used in the anode compartment, while anotherelectrolyte can be used in the cathode compartment. Generally, anyelectrolyte may be utilized in the cathode compartment which will notdestroy the ternary complex at the interface. A preferred electrolyte isthe monomer complex, which inhibits side reactions and produces onlyhydrogen at the cathode.

The operating conditions for the electrolysis reaction are not criticaland may vary over a wide range. The reaction temperature need only besuch that the reaction is effected in the liquid phase. Generally,however, temperatures will range from about -l0 C. to about +100 C.,preferably about 0 C. to 50 C., and still more preferably, at roomtemperature, i.e. about 18 to 26 C. Pressure may also vary widely, i.e.from about 0.5 atm. to about 10 atm. and preferably at atmosphericpressure.

The efiiciency of the current for the synthesis described in Ser. No.549,482 has been found to vary from 10 to 100% calculated on the basisof a requirement of two electrons for each original aromatic moleculeincorporated into the polymer in the center of a chain and one electronfor each such molecule on the end of a chain.

The present invention provides a means for overcoming these variationsand maintaining a current efficiency of to by using a potential of 1.15volts (vs. the saturated Cu:(h1 Cl electrode) for two to ten (preferably2) minutes to activate the electrode and then reducing the voltage to1.05 to 1.10 volts '(vs. the saturated CuzCu Cl electrode) for theremainder of the electrolysis in the synthesis of polyphenylene frombenzene.

The potential variation required can be advantageously and automaticallyaccomplished by controlling the current density of the Workingelectrode, While choosing the proper current density in accordance withthe following table.

Table I.Variation of current efficiency for poly (paraphenylene)formation as a function of current density Current density CurrentEfficiency,

1 Ratio of theoretical to actual quantity used.

Table II.--Infiuence on electrical yield of added solid aluminumchloride in the electrosynthesis of polyphenyl Gms of solid AlCl addedper Electrical yield,

hundred milliliters of anolyte percent 1 1 Electrolysis under optimumcurrent density galvanostatlc conditions. Theoretical yield of polymer0.20 gm.

As a further embodiment of this invention, the continuation of theproduct polymer with the A101 can be decreased and yet the requiredsaturation can be obtained by confining the solid AlCl inside a porouswalled reservoir added to the cell anode chamber. In this manner thedissolved AlCl is enabled to maintain its saturation level from theconfined reservoir, yet the AlCl is easily kept separate from thepolymer during separation of the latter by filtration from the cellelectrolyte.

The polymeric material may be recovered by hydrolyzing the complex phasewith ice and/or water. A twophase mixture will result: an inorganicphase comprising water, hydrogen halide, and aluminum halide; and anorganic phase comprising any unreacted excess aromatic, the aromaticfrom the ternary complex, and the polymer, either in solution orsuspended in the organic phase. The phases may then be separated by anyconvenient method, e.g. extraction, decanting, etc., and the polymerrecovered from the organic phase by extraction, sublimation,distillation, etc. Alternatively, and usually preferably, the polymermay be filtered or centrifuged from the liquid electrolyte, and theelectrolyte may then be returned to the cell for re-use. The solidpolymeric material thus separated may be treated with ice and/or wateras above.

The following examples will serve to further illustrate the process ofthis invention.

Example 1. Electrolysis of a complex phase of benzene andmesitylene:HCl:2AlCl In a glass U-tube, 50 ml. solution of the ternarycomplex mesitylene:HCl:2AlCl was saturated with 60 gm. of benzene andelectrolyzed between two platinum electrodes. A current of 100milliamps, corresponding to a current density of 8 milliamperes/squarecentimeter of electrode surface, was applied between the electrodes withcell voltage varying between 25 and 7 volts. The current was continuedfor about three hours and was continuously recorded. Using a referenceelectrode of Cu:'Cu Cl (saturated) in the electrolyte a potential of1.15 volts was found sufficient to activate the electrode. After twominutes the electrode potential was reduced below 1.11 and kept betweenthat potential and 1.07 for the remainder of the electrolysis. Thecontents of the cell were hydrolyzed with ice and water to destroy thecomplex phase. The organic products remaining after hydrolysis wereextracted and the starting materials, benzene and mesitylene, weredistilled away. From the remaining materials, 0.42 gm. of polyphenylenewere recovered. This corresponds to 96% of theoretical yield, assumingtwo electrons per benzene ring incorporated into the polymer. (Thisassumption corresponds to that generalized above, except that the endgroups are assigned two electrons as well as the center groups. This isunavoidable in this case because the ratio of end groups to centergroups is not known exactly. However, it is likely that there are atleast ten center rings for each end ring in the polymer, so that thisassumption leads to relatively small error.)

Example 2 Example 1 was repeated except that the solid AlCl was confinedinside a porous walled reservoir added to the cell anode chamber. Thepolymer obtained was filtered from the anolyte and was found to :bemixed with only 50% of its own weight of A1Cl as contrasted with morethan 1000% of its own weight without the use of the confining chamber.

The nature of the present invention having thus been fully set forth andexamples of the same given, what is claimed as new, useful and unobviousand desired to be secured by Letters Patent is:

1. In a process for preparing polymeric aromatic compounds whichcomprises electrolyzing a liquid solution comprised of a C to C aromaticcompound and a ternary complex having the formula:

wherein R is a C to C aromatic compound at least as basic as thearomatic in solution and X is chlorine or bromine, the solution alsobeing saturated with free AlCl the improvement which consists in makingthe potential of the working anode 1.15 (vs. the saturated Cu:Cu Clelectrode) for two to ten minutes at the beginning of the electrolysisthen reducing the electrode potential to 1.05 to 1.10 (vs. the saturatedCu:Cu Cl electrode) for the remainder of the electrolysis.

2. The process of claim 1 in which the potential variation is achievedautomatically by maintenance of a constant current density of 8 to 9milliamperes/ square centimeter.

3. The process of claim 1 in which the saturation with A101 is achievedby maintaining solid AlCl in a confined porous zone.

4. The process of claim 3 wherein the temperature is about 10 C. toabout +100 C.

5. The process of claim 3 wherein the aromatic in solution is selectedfrom the group consisting of benzene, biphenyl, naphthalene, alkylsubstituted benzenes, naphthalenes and biphenyls, and halo derivativesthereof.

6. The process of claim 5 wherein the aromatic in solution is benzene.

7. The process of claim 3 wherein R is selected from the groupconsisting of benzene, biphenyl, naphthalene, alkyl substitutedbenzenes, naphthalenes and biphenyls, and halo derivatives thereof.

8. The process of claim 7 wherein the ternary complex is mesitylene HCl2AlOl RD S- WILLIAMS, P im ry Examiner.

